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A Smarter, Simpler Approach to RF Power Solutions

Apr 15, 2020
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A Smarter, Simpler Approach to RF Power Solutions

Wolfspeed’s GaN on SiC solutions are revolutionizing the pre-driver, driver, and output stages of the RF power amplifier. Specifically, a new portfolio of high-electron-mobility transistors (HEMTs) now allow the transmitter chain to operate at a single bias voltage with higher efficiency, while delivering more power throughout a wide range of temperatures.

This is a major improvement because historically speaking, an S-band amplifier generally consisted of a few stages, all operating at different voltages, as shown in Figure 1.

Figure 1: Typical amplifier power lineup for S band

The Wolfspeed GaN on SiC portfolio contains components that are designed to work on a single bias voltage. This helps simplify design and lowers overall system costs. An example of a 50V design utilizing Wolfspeed components is pictured below in Figure 2.

Figure 2: Wolfspeed single bias voltage lineup

Now let’s take a look at the output stage section, specifically the CGHV31500F HEMT device. A typical HEMT device generally falls in the 2.7 — 2.9 GHz range, such as the 2729GN — 500V from Microsemi, which has its specifications shown in Figure 3.

Frequency
Pin (W)
Pout (W)
Id (A)
RL (dB)
Nd(%)
G(dB)
Droop (dB)
2700 MHz
35
562
2.45
-15
52
12.0
0.5
2800 MHz
35
537
2.25
-17
55
11.8
0.4
2900 MHz
35
575
2.05
-9
61
12.1
0.4
Figure 3: Power and efficiency ratings for 2729GN-500V by Microsemi (from datasheet)

If you compare these parameters with the plot shown in Figure 2 of the CGHV31500F datasheet, along with its corresponding table (shown below in Figure 4), then it’s apparent the same class of device delivers a higher output power at an increased efficiency even with a wider bandwidth.

CGHV31500F Output and Drain Effeciency vs Frequency

Parameter
2.7GHz
2.9GHz
3.1GHz
Units
Output Power
650
705
604
W
Gain
12.1
12.5
11.8
dB
Drain Effciency
70
68
58
%
Figure 4: CGHV31500F output power and drain efficiency versus frequency (taken from datasheet)

Furthermore, when comparing these two sets of data for power output, we can observe a power increase between 15% and 20% with the Wolfspeed component. And it does this with increased reliability and thermal performance. Simply put, this is the Wolfspeed advantage.

Wolfspeed offers three levels of customer support for design assistance and integration: RF model simulations (shown in Figure 5), design files and references, and standard test fixtures for all devices, as well as custom fixtures for varying frequency bands. All simulation files, gerbers, schematics, and BOMs for reference designs (such as the CGHV35120F-AMP2 Demonstration Circuit shown in Figure 6), are available upon request. This helps ensure first-pass success through the design phase which, in turn, improves overall speed to market.

Figure 5: Wolfspeed RF model simulation examples
Figure 6: Wolfspeed CGHV35120F-AMP2 Demonstration Circuit

One question that does come up: how does reliability improve with higher power and smaller form factor? Wolfspeed has achieved this by focusing on improving the efficiency to allow for the junction temperature to operate in a safe range. By performing finite element analysis (FEA) and modeling temperatures at the junction (as shown in Figure 7) and using this information in the design process, it’s been shown that a maximum temperature of less than 225°C can be achieved. The industry standard is 150°C, as seen on LDMOS and GaAs devices, however this GaN on SiC family of transistors can offer a mean-time-to-fail (MTTF) value of over 10 years at this temperature.

With over 206 billion hours of fielded devices and 15 years of production, this has equated to a failure in time (FIT), or failure rate, of less than two!

Figure 7: Wolfspeed FEA on junction temperature

This is in direct contrast of competitors who utilize IR temperature readings which give an optimistic view of the junction temperature resulting in lower reliability. Figure 8 summarizes the reliability differences when compared to a leading competitor.

 
Wolfspeed
Leading GaN-on-SiC Competitor
Ɵjc
Based on FEA
Based on IR
Tjc, max
225C
200C
MTTF @ 200C
1 x 107
1.13 x 106
MTTF @ 225C
2 x 106
1.6 x 105
Figure 8: Reliability comparison between Wolfspeed products and industry-leading competition

And how do you know that you’re actually getting higher power and efficiency? Wolfspeed has spent quite a bit of effort characterizing and validating peak power and efficiency of the GaN field-effect transistors (FETs), as shown in the application note (APPNOTE-017 Rev. A). Load-pull measurements were taken for a 100W and 200W Wolfspeed transistor at optimal impedances across multiple frequencies and very closely follow the modeled values. The results showed that Wolfspeed’s large signal models accurately demonstrate the performance of these devices.

o summarize, Wolfspeed’s portfolio of L-band and S-band components have some very distinct advantages over similar, industry-leading devices, including higher power output, better efficiency, and greater frequency range, all in a smaller, simpler package. This allows for a lower overall system cost and better thermal performance, resulting in increased reliability for the life of the device.

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